1. Field of the Invention
The present invention relates to a production method for silicon single crystals. More specifically, the present invention relates to a production method for silicon single crystals based on the Czochralski method (hereinafter referred to as the “CZ method”) which can prevent the occurrence of dislocations during separation from the silicon melt, thereby improving the shape of the separation surface of the produced silicon single crystal thus produced.
2. Background Art
The CZ method includes the steps of heating raw material polycrystalline silicon in a crucible to produce a silicon melt, immersing a seed crystal in the silicon melt, and pulling up a silicon single crystal by gradually rewinding a wire attached to the seed crystal.
At the beginning of crystal growth, a cone section with a gradually-increasing cone diameter, i.e. an increasing-diameter section, extending from the contact surface with the seed crystal is grown, and then growth of a straight-body section, i.e. a constant-diameter section, begins when the crystal diameter has reached a target diameter. The silicon single crystal is finally separated from the raw material melt when the length of the straight-body section reaches a predetermined length.
In a CZ conventional production method, the step of separating the silicon single crystal from the raw material melt tends to cause a sudden temperature drop at the lower end of the silicon single crystal, which is the separation surface from the silicon melt. This may result in a considerably-reduced crystallization ratio (yield) due to the occurrence of slip dislocations within the silicon single crystal after being lifted up from the melt.
To avoid such dislocations, a conventional production method further includes a step of gradually narrowing the diameter of the silicon single crystal after the growth of the straight-body section before separating the silicon single crystal from the raw material melt, so that the contact surface between the silicon single crystal and the raw material melt will be small enough to avoid the occurrence of dislocations. This narrowed section is generally called a tail section. However, the tail section is known to cause a decreased yield ratio since its crystal diameter is smaller than a desired value, and therefore this section is not considered to be a product.
Accordingly, it would be desirable to reduce or omit entirely the step of producing the tail section, in order to increase productivity of silicon single crystals.
A number of techniques for reducing or omitting generation of the tail section already exist. One technique includes steps of lifting up a crucible containing the raw material melt at the same lifting speed as that of the silicon single crystal after the growth of the straight-body section, rendering the bottom surface of the silicon single crystal downward protruding, and thus suppressing dislocations during separation of the silicon single crystal from the raw material melt. (See Japanese Patent Application Publication No. 2007-176761).
In recent years, silicon wafer diameters have become increasingly lager, and 300 mm wafers are now the main products. The production of a silicon single crystals with large-diameters generally involves a strengthened natural convection current within the crucible due to the increased weight of the silicon melt, which tends to cause dislocations and a deformation of the silicon single crystal. Therefore, various approaches have been made to control the natural convection current, for example by applying a horizontal magnetic field to the silicon melt, for the purpose of suppressing dislocations and deformation.
However, application of a horizontal magnetic field adversely causes a decreased temperature difference between the center and the periphery of the phase boundary of the silicon single crystal, compared to its absence, which can prevent the silicon single crystal from forming a downward convex shape at the phase boundary, which disrupts the omission of the tail section intended to suppress the occurrence of dislocations during separation of the silicon single crystal from the melt, while suppressing dislocations and deformation in the straight-body section.